Final answer:
During muscle relaxation, calcium ions are actively transported back into the sarcoplasmic reticulum, which is crucial for lowering the calcium levels in the sarcoplasm and enabling the muscle to relax. The correct option is A.
Step-by-step explanation:
Discussing the fate of calcium during muscle relaxation, calcium ions are actively transported into the sarcoplasmic reticulum (SR). This process is an ATP-dependent mechanism in which calcium pumps work to move calcium from the sarcoplasm back into the SR, lowering the calcium ion concentration in the sarcoplasm and leading to muscle relaxation.
It is important to note that the calcium does not sequester in the T-tubules or get metabolized by enzymes, nor does it diffuse out of the muscle fiber into the surrounding tissues during relaxation.
Instead, these ions are specifically pumped back into the sarcoplasmic reticulum, which is a specialized endoplasmic reticulum found in muscle cells, responsible for the storage of calcium ions needed for contraction and relaxation cycles.